I don't know where you live but an on line recycle site called freecycle would be a good place to start if they have people close willing to recycle. Put a request for some specific books or perhaps a list of age appropriate books and see what you get.
Put a free ad in one of your local papers asking for donations but be specific or you will get things you don't want.
Ask the local librarian when they clear their shelves of books. Most libraries have a clean out once a year and sell off the books inexpensively.
Ask any young teens you know if they have books they have outgrown that they could donate.
Contact publishers of the books you'd most like to have and ask if you could be on their review list. If you are accepted and keep to the bargain they will send you new books, you write a review and get to keep the book.
Hope this helps!
Marilyn